Eco-update

Volatile Chemical Signals from Damaged Plants Warn Neighbors about Herbivore Attacks

Animals often use highly specific signals to warn their herd about approaching predators. Surprisingly, similar behaviors are also observed among plants. Tokyo University of Science researchers have discovered one such mechanism. Using Arabidopsis thaliana as a model system, the researchers have shown that herbivore-damaged plants give off volatile chemical “scents” that trigger epigenetic modifications in the defense genes of neighboring plants. These genes subsequently trigger anti-herbivore defense systems.

Prior studies have shown that when grown near mint plants, soybean and field mustard (Brassica rapa) plants display heightened defense properties against herbivore pests by activating defense genes in their leaves, as a result of “eavesdropping” on mint volatiles. Put simply, if mint leaves get damaged after an herbivore attack, the plants in their immediate vicinity respond by activating their anti-herbivore defense systems in response to the chemical signals released by the damaged mint plant. 

“In this study, we hypothesized that histone acetylation, or the so-called epigenetic regulation, is involved in the phenomenon of resistance development,” explains Dr. Gen-ichiro Arimura, Professor at the Tokyo University of Science and one of the authors of the study. Their findings have recently been published in the journal Plant Physiology.

First, they exposed the plants to β-ocimene, a volatile organic compound often released by plants in response to attacks by herbivores like Spodoptera litura. Next, the researchers tried to determine the exact mechanism of action of volatile-chemical-activated plant defense.

The results were interesting ― defense traits were induced in Arabidopsis leaves, presumably through “epigenetic” mechanisms, which refer to gene regulation that occurs because of external environmental influences. In this case, the volatile chemicals released by the damaged plants enhanced histone acetylation and the expression of defense gene regulators, including the ethylene response factor genes ERF8 and ERF104. The team found that a specific set of histone acetyltransferase enzymes (HAC1, HAC5, and HAM1) were responsible for the induction and maintenance of the anti-herbivore properties. 

The researchers are ecstatic about their discovery of the role that epigenetics has to play in plant defense. According to them, the communication between plants via volatile compounds (known as the “talking plants” phenomenon) can potentially be applied to organic cultivation systems. This can increase the pest resistance of plants and effectively reduce our massive dependence on pesticides.

“The effective use of plants’ natural survival strategies in production systems will bring us closer to the realization of a sustainable society that simultaneously solves environmental and food problems,” concludes Dr. Arimura.

Benefits of Cover Crops Extend to Dry Areas

Cover crops do far more than cover soils. They provide an array of benefits, such as the ability to reduce soil erosion and increase soil health. They can attract pollinators, repel pests, turn into green manure, or be used as feed for livestock.

A new study shows that the benefits of cover crops extend even into semi-arid areas. This review was recently published in the Soil Science Society of America Journal.

“Much of the research data we have on cover crops is from regions with high precipitation,” says Humberto Blanco, lead researcher at the University of Nebraska-Lincoln. “So, questions remain about ecosystem services provided by cover crops in drier regions.”

Some skeptics have argued that growing cover crops in more arid areas could use too much water and could in turn reduce subsequent food crop yields. But the research concludes that this isn’t necessarily the case.

“We found that cover crops can improve most ecosystem services in water-limited environments,” says Blanco. “In the majority of cases, these improvements come without negative effects on food crop yields.”

To determine how well cover crops work in semi-arid areas, Blanco and colleagues assembled and analyzed the limited number of studies on cover crops in dry regions. They emphasized studies focusing on the semi-arid Great Plains in the United States.

The researchers looked at cover crops in connection with several ecosystem services. These included the amount of organic carbon in soils, soil microbial properties, weed management and food crop yields, among others.

One of the key soil features the researchers focused on was soil organic carbon. “Soil organic carbon is the catalyst for many other changes in soil properties and soil services,” says Blanco. “Soils in water-limited regions are often low in organic carbon.”

The researchers found that in dry areas, cover crops increased soil organic carbon levels close to 60 percent of the time.

“This accumulation of organic carbon is critical to these soils,” says Blanco. That’s because soil organic carbon is the food source for many soil organisms, like microbes. Ultimately, these soil organisms play a vital role in maintaining healthy, fertile soils.

Cover crops also suppress weeds in dry areas. This is especially important because several weed species are resistant to current herbicides. The suppression of weeds by cover crops has a knock-on effect of increasing water conservation and preventing soil erosion.

“Herbicide-resistant weeds can lead to tillage of typically no-till systems,” says Blanco. “This can reduce the water-conservation ability of those agroecosystems.” Tilling can also make soils more susceptible to erosion.

Cover crops also provide food for livestock in dry areas. “Grazing or haying cover crops can improve net returns without negating benefits to soils,” says Blanco. That’s because even when grazed, a significant portion of cover crops remain on fields. Also, cover crop roots persist even when grazed, holding soils together and providing many benefits.

The study found that cover crops can reduce food crops’ yields in some cases. These instances typically coincided with intermittent drought conditions. Water availability for cover and food crops decreased during these years.

“Adapting crop rotations and cover crop use to accommodate weather conditions is critical,” says Blanco. “Farmers in drier areas may not be able to plant a cover crop every year. They can target wet years when cover crops can be successful.”

“Long-term research is critical to identify the enduring effects of cover crops,” Blanco says. “Yet, long-term research data for cover crops in arid and semi-arid areas are virtually absent in the literature.”

Agricultural Fungicides May Be Driving Antimicrobial Resistance

New research from the University of Georgia has shown, for the first time, that compounds used to fight fungal diseases in plants are causing resistance to antifungal medications used to treat people.

The study focused on Aspergillus fumigatus, the fungus that causes aspergillosis, a disease that causes life-threatening infections in 300,000 people globally each year. Published in G3: Genes, Genomes, Genetics, the study linked agricultural use of azoles — compounds used to fight fungal diseases in plants — to diminished effectiveness of the clinical azoles used to treat fungal infections in patients.

“Our results show that resistance to the compounds used to combat fungal infections in humans is developing in agricultural environments,” said Marin T. Brewer, a corresponding author of the study and an associate professor of mycology. “The samples that we collected in agricultural settings were resistant to both the azoles used in the environment and the clinical azoles used to treat people.”

Fungi can be a menace for both people and plants, causing over 1.5 million human deaths annually and crop losses of 20 percent. It’s not unusual to find A. fumigatus in the environment. It’s airborne and it’s everywhere. Most people breathe it in without problem, but it can cause serious infections in people who have weakened immune systems.

When they’re infected by a strain of the fungus that’s resistant to agricultural azole fungicide, the clinical azole drugs used in healthcare are also ineffective.

“Azole-resistant A. fumigatus is widespread in agricultural environments and especially things like compost,” said Michelle Momany, a corresponding author of the study. “Someone who is immunocompromised and at risk for fungal infections should be very cautious in those settings.”

The team collected samples of soil, plant material and compost from 56 sites in Georgia and Florida. Most of the sites had recently been treated with a mix of fungicides — including azoles and other fungicides that are only used in agriculture, not in patients. But two of the sites were organic and hadn’t used fungicides in over a decade.

After recovering strains of A. fumigatus, the researchers found 12 that were highly resistant to azoles used in agriculture and medicine. The 12 strains also exhibited high levels of resistance to two non-azole fungicides that are not used to treat people.

The researchers used whole genome sequencing to create a genetic family tree for A. fumigatus strains from the environment and from patients. They found that the mechanisms of azole resistance they identified in the strains from agricultural environments matched what they saw in patients. The azole-resistant strains from patients were also resistant to the non-azole fungicides that are never used in people, showing that these strains had been in agricultural environments before the patients were infected.

“The strains that are from the environment and from people are very closely related to each other,” Brewer said. “It’s not like there are different strains that are developing resistance in people and in the environment. It’s all the same. So people who have these infections that are resistant have likely acquired them from the environment.”

Of the 25 multi-azole-resistant strains included in the study, eight from agricultural environments and 12 from patients were also resistant to the non-azole agricultural fungicides. These multi-fungicide resistant strains were from agricultural settings in the U.S. and India and clinical settings in the U.S., the Netherlands and India.

“A. fumigatus that is resistant to multiple fungicides is all over the globe, both in the environment and the clinic,” Momany said.

“This emergence severely limits the usefulness of fungicides to manage plant pathogens while still preserving the clinical usefulness of azoles,” Brewer said. “We urgently need effective agricultural fungicides that aren’t toxic to the environment that do not lead to the rapid development of widespread resistance in the clinic.”

Midwestern U.S. Has Lost 57.6 Billion Metric Tons of Soil Due to Agricultural Practices

A new study in the journal Earth’s Future, led by the University of Massachusetts Amherst, shows that since Euro-American settlement approximately 160 years ago, agricultural fields in the midwestern U.S. have lost, on average, 2 millimeters of soil per year. This is nearly double the rate of erosion that the USDA considers sustainable. Furthermore, USDA estimates of erosion are between three and eight times lower than the figures reported in the study. Finally, the study’s authors conclude that plowing, rather than the work of wind and water, is the major culprit.

“A few years back, my wife and I were at a wedding at a pioneer Norwegian church in Minnesota,” says Isaac Larsen, professor of geosciences at UMass Amherst and one of the paper’s co-authors. “After the ceremony, I walked over to the edge of the churchyard, which was surrounded by cornfields, and was shocked to see that the surface of the field was a few feet lower than the surface of the never-tilled churchyard. I began to wonder why.”

A few years later, and Larsen found himself standing in central Iowa on the “escarpment,” or drop-off, separating a native prairie from a field of soybeans.

The team worked extensively with the Iowa Natural Heritage Foundation and other organizations to pinpoint the few remaining pockets of original, never-farmed prairie. They then reached out to the farmers whose land abutted the prairies, asking them for permission to survey their fields. They wound up with twenty sites, the majority of them in central Iowa, with a few in Illinois, Minnesota, South Dakota, Kansas and Nebraska. 

Using an extraordinarily sensitive GPS unit that looks more like a floor lamp than a hand-held device, the team walked dozens of transects, or perpendicular routes, across the escarpment, from the untouched prairie to the eroded farm field, stopping every few inches to measure the change in altitude. They did this hundreds of times throughout the summers of 2017, 2018 and 2019.

Once they had their raw data, the team used historical land-use records and cutting-edge computer models to reconstruct erosion rates throughout the Midwest. What they discovered is that Midwestern topsoil is eroding at an average rate of 1.9 millimeters per year. Put another way, the authors estimate that the Midwest has lost approximately 57.6 billion metric tons of topsoil since farmers began tilling the soil 160 years ago. And this is despite conservation practices put in place in the wake of the Dust Bowl in the 1930s.

It’s also clear that much of the erosion is due to tillage. “The modeling that I do shows that tilling has a ‘diffusive’ effect,” says a team member. “It melts the landscape away, flattening higher points in a field and filling in the hollows.” But because the USDA does not explicitly include such “tillage erosion” in its own analysis, it has underestimated the rate of erosion currently at work in the heartland.

“As erosion degrades our soils, it reduces our ability to grow food,” says Larsen. “Combine this with increasing global population and climate stress, and we have a real problem.” The team suggests that more sustainable practices, such as no-till farming and soil regeneration, “will likely be required to reduce soil erosion rates in the Midwest to levels that can sustain soil productivity, ecosystem services and long-term prosperity.”